VTOL Technologies Managing Director, Ashley Bryant, asks the question, is there a need for new low-altitude VTOL UAV concepts for military and commercial applications?
Does the optimum VTOL UAV architecture already exist for urban, mountainous and maritime UAV operations or should there be further investment in the development of optimised VTOL UAV platforms, to specifically address these very low and low-altitude environment requirements?
Market research carried out by Moiré Inc. (a US company specializing in business and technical consulting for unmanned vehicles for aerospace, marine, & military systems) back in 2005, would suggest that once UAV operations have been fully integrated and established as part of normal flight operations within the current manned aircraft infrastructure, (i.e. the current challenges of flying UAV’s within regulated airspace have been overcome), then over 84% of operational UAV’s will be flying at very-low altitude (less than 1000’) with a further 12% flying at low altitudes. If this is correct, then there is an important question to ask, where will these very low-altitude UAV’s be operating?
CHANGING WORLD DEMOGRAPHICS
A good place to start is by looking at the world’s changing demographics. Today, over 50% of the world’s population is living in urban environments, whilst the figure for the North American and European continents is already at 70% and these numbers are continuing to rise. Urban environments are often congested at ground level and the quickest initial access for primary-response teams is from the air, especially for a rapid assessment of the situation on the ground, delivering amongst other benefits, improved decision-making over which ground personnel should go where and by what route. Warfare too continues to evolve, with operations in both Iraq and Afghanistan often being conducted in and around urban settlements, from which the insurgents obtain both protection from buildings as well as by mingling with the local population.
Given this situation, is there an advantage deploying very-low altitude UAV’s? One of the most interesting and potentially compelling arguments is the parallel between UAV operations and the battle for supremacy between the mainframe computer and the PC in the early 1990’s. As the PC became more powerful and more affordable, individuals wanted greater local/personal flexibility, including the ability to select which software applications they ran, when and how. Furthermore, as software packages increased in terms of application choice and functionality, purchasing a much lower cost, portable computer provided individuals with a more flexible tool that could better address their personal needs and requirements.
There is a clear parallel here between the technology advances that made possible the shift from mainframes to a distributed mainframe/PC architecture. Many of the core technologies required for very-low altitude UAV platforms and their payloads are being miniaturised and manufactured at ever lower costs, as a result of the demand from volume markets in the commercial sector, such as the mobile phone and satellite navigation industries. Examples of such core technologies include miniaturised EO optics, inertial systems, GPS chipsets, smaller, lighter and more powerful laptop PC’s for ground station control as well as the mobile phone communications infrastructure itself.
So if advancements in technology have the potential to deliver miniaturised, lower cost, very-low altitude ISTAR capabilities, what difference exists, if any, between very low-altitude and high-altitude UAV operations, putting to one side any differences based purely on technology miniaturisation and cost.
DIFFERENCES BETWEEN VERY-LOW & HIGH ALTITUDE OPERATIONS
High altitude UAVs are, by their very nature large and costly assets to acquire and operate, (some platforms being larger than a Boeing 737). Flying at high altitude requires costly payload optics and communications infrastructure. Having said that, there are clear advantages too. They have the longest endurance of all UAV’s and so long as lower altitude cloud cover is not an issue, also have good line of sight for ground-based targets. The platform is also very stable, smooth flight paths can be planned and programmed hours and even days in advance of an operation, with multiple payloads provided on a single platform.
Since high-altitude UAV’s will typically operate from bases thousands of miles from the target location, they consume a far greater assortment of bandwidth assets, ranging from satellite communications to secure ground-based networks, than much smaller, locally deployed UAV’s. Increasing the number of high-altitude assets operating over long distances can quickly saturate bandwidth resources. The issue of bandwidth is a serious one, with bandwidth restrictions today being one of the most important limiting factors in the growth of UAV operations.
By comparison, very-low altitude UAV’s can be very small indeed, weighing less than 4kg / 8.8lbs, with a typical platform fitting within a 1 to 1.5m square footprint. They can operate below controlled air-space and potentially obstructing cloud cover. If they crash, they are likely to generate far less collateral damage than larger platforms. Not only that, but if destroyed, they are easier and less costly to replace and are available in far higher numbers than the strategic assets. Being highly portable, they can be used for point problems as and when they occur, rather than operating at a strategic and highly planned mission level. There is also the added advantage of hot-swappable payloads for multiple operations. So long as their required range of operation is short, bandwidth considerations are reduced (operating in a cell network configuration) and with the potential for swarming.
So, if very-low altitude operations have certain advantages, what are the challenges operating at such low altitudes above ground level and do we have the most efficient UAV platforms that can efficiently operate under such conditions?
The smaller the platform, the shorter its endurance and the more sensitive it is to disturbances (gusts, etc), not forgetting to point out that it is easier to identify and see at very-low altitude and hence shoot down as a result of a more prominent noise signature. Operating at such low altitudes brings other challenges such as additional safety issues, since the time available to correct flight problems is much shorter. At high altitude, platform manoeuvrability is not a problem, but the lower the altitude a UAV flies at, the more that platform manoeuvrability plays an important role. The lower the altitude, the easier it is for ground objects to obstruct the line of sight of the surveillance equipment on board the air vehicle and the greater the requirement for increased and more accurate manoeuvrability.
This line of sight issue is especially challenging for very-low altitude fixed-wing UAV’s, since these platforms cannot easily position themselves directly above their targets as a result of minimum flight speeds and in many cases, certainly not for sufficient time to capture the required information or successfully track moving targets in built-up areas. Some sort of very low speed flight and hover capability is required to address this issue. This could be achieved using rotorcraft UAV’s. However, there is still the important issue of exposed rotor blades and hence safety issues in urban environments. On top of all this, deployment and recovery challenges also need to be addressed. The UAV must be reasonably close to its target at the point that it is deployed, if it is to be able to get on station and stay on station for an effective period of time within its range, endurance and top speed limitations.
The other really big challenge operating at very-low altitude is gusting. Buildings or other structures stand in the way of surface winds and therefore create by their very nature, turbulence or gusting. The smaller the aircraft; the greater typically is the challenge of remaining stable in such unpredictable conditions. This is further compounded by the fact that for surveillance, reconnaissance or search & rescue purposes, slow-speed flying or even hovering can be a real advantage, but demands even greater gust resistance or gust insensitivity. Fixed-wing UAVs are unable to fly slowly, having a minimum forward flight speed, whilst helicopters have exposed rotor blades and can become unstable, if they approach a building or structure within 1½ rotor diameters or less. Operating in urban canyons only exacerbates these challenges and yet it is a growing military requirement.
SIMILAR OPERATIONAL REQUIREMENTS
These ‘urban-operations’ challenges (endurance, manoeuvrability, gust-resistance, operating from a small footprint, safety, etc) are replicated for both mountainous and maritime operations. Search & rescue operations in mountainous regions drive the requirement for flying close to cliff faces as well as through mountain gulleys and valleys, which generate similar turbulence conditions and up drafts as those found in urban environments. Likewise the same can be said operating from ships in maritime environments. Landing on the deck of a ship has multiple challenges; cross-winds will create an updraft from the ships hull, generating gusts around the landing area, whilst the slow pitching and rolling of the ships’ deck adds further complexity. The smaller the UAV, the more challenging these landings become. Operating from smaller vessels and the frequency of the ships’ pitching and rolling increases, whilst the deck footprint for landing is arguably going to be smaller.
If these are the challenges, do current very-low altitude UAV architectures fully match these requirements and challenges?
Fixed-wing UAVs have good endurance, but are hampered by a lack of VTOL capability, low-speed flying, manoeuvrability, gust sensitivity and a requirement for runways or launch & recovery mechanisms such as netting, parachutes, etc.
In Afghanistan for example, small UAV’s are being recovered by stalling the aircraft shortly before landing, because the terrain is so rocky (just like a moonscape) and a conventional fixed-wing aircraft landing is therefore impossible. The net result is that over time, many of these fixed-wing UAV’s become unusable, reducing the overall mission operational capability. It is not unknown for more UAV losses to occur than anticipated and spares to be extremely hard to come by, certainly within the required timescales.
Rather than attempting a hand-launch, or even setting up additional equipment for a catapult launch, VTOL provides an instantaneous launch capability, wherever the equipment is operated from, even within protected compounds. But arguably, a VTOL platform is even better equipped to address the requirement for safe recovery of the platform post operation, since it can land on most terrain without undue problems.
Combinations of the advantages of fixed-wing designs coupled with the advantages of rotor-craft/helicopter designs have been developed. The V22 Osprey and Bell/Textron Eagle Eye have gone a long-way in combining these advantages, but there still remains the issue of technical complexity (centralised propulsion unit, drive shafts and gear boxes through the wings, variable pitch propellers, time between servicing, operational costs and developing smaller sizes of platform that are gust insensitive).
Although rotorcraft have all the advantages of VTOL operations, both standard helicopters and tilt-rotor craft still have a real challenge when it comes to operating at very-low altitude, delivering safety and endurance as well as low cost operation. The majority of these issues are down to the large, exposed rotor blades as outlined earlier in this article.
Can the advantages of rotorcraft be combined with fixed-wing to produce a higher performance UAV?
One concept that has combined the advantages of the helicopter with those of the fixed-wing is a unique ‘thrust-vectoring flying-wing’ design under development by VTOL Technologies Ltd. The really new element is the combination of an advanced flying wing aerofoil coupled with four thrust-vectoring propulsion units delivering rapid pitch & roll control to counter gusts, but also providing rapid acceleration and deceleration when appropriate, together with safer operation. Light impact with buildings/structures will not bring the platform down, due to the smaller, shrouded propulsion units.
Some of the advanced features include almost instantaneous stall recovery, gust insensitivity, reverse thrust to enable the platform to be ‘sucked down’ onto the deck of a ship pitching in heavy seas and minimum power to rotate the thrust-vectoring propulsion units. Loitering into wind, keeping the vehicle airborne with minimal power, just like sea birds, is also a feature.
In the event of power failure in one or two of the propulsion units, the vehicle can still remain airborne and fly at close to its cruising speed, whilst total propulsion unit failure poses fewer risks than for rotary-wing craft such as helicopters by flying with a shallow glide-angle, rather than descending to the ground under auto-gyration.
This unique patent-pending design has 3 to 4 times the endurance of current VTOL UAV’s carrying an equivalent power source and payload but delivering significantly faster cruise speeds, whilst the same comparisons with current fixed-wing UAV’s demonstrate more than twice the endurance, but with the added advantages of VTOL, high manoeuvrability, low-power loiter and a continuous speed range, which enable this platform to readily operate in urban canyons, still a huge challenge for current UAV systems. The thrust-vectoring propulsion unit design enables the UAV to be launched and recovered from moving vehicles or ships without the need for ancillary equipment.
With advances in specific green fuel-cell technologies, these endurance ratios could be quadrupled within the next couple of years, delivering the much needed longer endurance figures, not possible with today’s small, low-altitude UAV’s
Are there any limitations regarding this new concept?
At first glance, one might argue that the location and crank-arms of the ducted-fan propulsion units might be a weak point in the design, but then the Bell V22 Osprey achieved similar thrust-vectoring with only a single tilt-rotor at the end of each wing, whilst the Bell D2127 / X-22 achieved a similar thrust-vectoring configuration with very large ducted-fans, the aircraft being designed for VTOL transport operations. Maybe, at larger scales, alternatives to the ducted-fan propulsion system proposed at small scales could be replaced with alternative thrust-vectoring propulsion technologies.
In conclusion, will this or other advanced very-low altitude UAV architectures succeed?
Only time will tell?